Updating a Mac model released before 2016 to the latest macOS version, macOS Sequoia (macOS 15), is generally not possible, as these older models are not officially supported. Apple's compatibility list for macOS Sequoia includes:
MacBook Pro: 2018 and later
MacBook Air: 2020 and later
iMac: 2019 and later
iMac Pro: 2017 and later
Mac mini: 2018 and later
Mac Studio: 2022 and later
Mac Pro: 2019 and later
Therefore, Mac models from 2015 or earlier are not eligible for the macOS Sequoia update.
Apple Support
However, if your Mac isn't compatible with macOS Sequoia, you might be able to upgrade to an earlier version of macOS instead.
Apple Support
To determine your Mac's model and year, click the Apple menu in the top-left corner of your screen and select "About This Mac." This information will help you identify which macOS versions are compatible with your device.